The present invention relates to cinematographic apparatus in general, especially to motion picture projectors, and more particularly to improvements in elastic compensating members or dampers which are utilized in such apparatus to engage motion picture film intermediate a continuously rotating device (such as the supply reel in a motion picture projector) and a device (such as the film gate in a projector) where the film is advanced stepwise or intermittently, normally by the claw of a pulldown or intermittent.
Presently known dampers between the supply reels and film gates of motion picture projectors operate satisfactorily as long as the film coming from the core of the supply reel offers a predictable constant resistance to forward movement under the action of the pull-down. As a rule, a conventional damper is fully capable of compensating for the fact that the supply reel rotates continuously whereas the pull-down advances the film stepwise so that each film frame is at a stand still during the interval of projection of its image onto a screen or the like. To this end, the damper undergoes repeated elastic deformation at one or two points, first as a result of increasing tension when the film is being pulled by the claw of the pull-down and thereupon due to its elasticity as the supply reel rotates by inertia while the pull-down performs a return stroke.
However, a conventional damper does not operate satisfactorily when the tension of film fluctuates for reasons other than the fact that the pull-down draws the film intermittently off a continuously rotating supply reel. For example, the tension of motion picture film will fluctuate often within a rather wide range, if the core of a supply reel carries a large roll of convoluted film and the supply reel is out of round or rotates with a certain amount of radial and/or axial play. The just mentioned unpredictable random fluctuations of the film tension result in the generation of noise and in blurring of projected images because the frames are not in exact register with the gate during projection of their images onto a screen.
It was already proposed to utilize additional or auxiliary dampers which are intended to compensate for randomly developing and unpredictable changes in tension of motion picture film between the supply reel and the film gate of a motion picture projector. However, the characteristic vibration of such auxiliary dampers is different from that of customary elastic dampers. Therefore, whenever an auxiliary damper becomes effective (i.e., when the tension of film changes for one or more reasons other than stepwise film transport by the pull-down), it is likely to affect the characteristic of the customary damper and/or to exert an adverse influence directly upon the motion picture film. Moreover, the manufacture, mounting, maintenance and inspection of auxiliary dampers contribute to space requirements as well as coat of cinematographic apparatus.